The present invention relates to video keyers, and more particularly to a component video accumulation effect for combining, or accumulating, key sources and video fill signals to reduce the number of mixers required, the effect producing a predictable visual effect where the different component video signals overlap.
An output video signal that is displayed on a television monitor may consist of a combination of different video signals from different sources. For example a foreground video signal may be cut to appear in front of a background video signal and characters from one or more character generators may be cut to overlay the entire video image. For each video source there is a video signal and an associated key signal that determines where each video signal appears in the output video signal. Conventionally each combination of two video sources requires a separate mixer with the appropriate key signal. In composite television a technique has been developed to accumulate one or more key sources and video fill signals so that only a single keying mixer is required to insert them into a background video signal. This is accomplished by the use of a non-additive mixer (NAM) having the key signals or video signals as inputs to produce a single output key signal and video signal that is input to the single keyed mixer for combination with the background video signal. This is possible because the entire composite video signal is positive with respect to a blanking level. However in component video, although the luminance component is positive with respect to the blanking level, the chrominance components may have either a positive or a negative value with respect to the blanking level. Therefore straight forward non-additive mixing of the bipolar chrominance components is not possible.
In the composite video arena when the video signals from different sources are accumulated at an overlap area between the respective key signals, improper color gamut signals may occur resulting in an unpredictable color result. This occurs because the video signal having the highest instantaneous voltage level is displayed, and where the voltage levels are close together the subcarrier component may alternate between the two video signals.
Therefore what is desired is a method of accumulating component chrominance signals so that a single key mixer may be used for a multiple video fill mix, and also so that a predictable, pleasing and legal color results when the video signals overlap.